Vehicles include an engine, a transmission, and drive wheels. More recently, it has been proposed to use what is known as a hybrid transmission wherein the vehicle powertrain includes an engine, a transmission, and drive wheels with the transmission incorporating one or more electric motors which assist in driving the vehicle or can drive the vehicle while the internal combustion engine is inoperable.
These types of powertrains have the benefit in that they reduce vehicle emissions and improve the overall fuel economy of the vehicle. This is mainly accomplished by allowing the engine to stop or discontinue operations under certain conditions.
In at least one hybrid transmission or powertrain, a parallel hybrid transmission is incorporated. In the parallel hybrid transmission, power is supplied by the internal combustion engine and by the electric motors, which are independently connected to drive members. Power from the vehicle engine is delivered to the transmission, which transfers the power to the drive wheels. One or more motor/generators are incorporated into the transmission and connect through energy storage devices such as batteries. The energy storage device can either accept power from or supply power to each of the motor/generators.
An onboard computer is used to synchronize the power flow from the engine and/or the motor/generators to the transmission output. In order to provide better efficiency over a wide range of applications, it is desired to permit discontinuation of operation of the internal combustion engine during certain modes of operation such as coasting, low speed operation, and stopping in traffic, or for other purposes. By discontinuing the engine operation during these instances, the inefficiency and emissions of the vehicle are greatly reduced. However, when the engine operation is discontinued, the control fluid for the transmission is also discontinued which can result in slower startup operation of the transmission and/or require one of the electric drive motors to also drive a pump which would require the drive motor to be operated during these operating conditions. It would be of great advantage to maintain the range clutch or torque-transmitting mechanism of the transmission engaged during these operations when the engine operation is discontinued.